Gold alloys have long been used for the preparation of dental restorations, such as dentures and dental prosthetic parts.
For aesthetic reasons, porcelain-fused-to-metal construction has become increasingly popular.
The invention of the porcelain-fused-to-metal processing technique (Weinstein, et al. U.S. Pat. No. 3,052,982) was readily accepted in the marketplace and became the procedure of choice because both function and aesthetics were addressed by it. This technique involves casting a thin metal alloy substructure and baking successive layers of porcelain onto the alloy, until the proper bulk and form is achieved. The porcelain provides the life-like appearance of natural teeth, while the metal alloy substructure provides the necessary strength and durability for repeated mastication.
The requirements for alloys used in porcelain-fused-to-metal construction, in accordance with the prior art, were thought to be:
1. High strength. For example, U.S. Pat. No. 3,981,723 teaches an alloy which has strength values approaching ADA Spec. No. 5, or Type IV dental casting gold alloys (Type IV is the highest strength requirements).
2. Thermal expansion coefficient "matched" to the porcelain. For example, U.S. Pat. No. 5,423,680 discusses the range of ceramic thermal expansion coefficients existing in the marketplace and teaches how to increase the thermal expansion coefficients of alloys to high levels.
3. Bondable to porcelain. The necessity to have an oxide present on the metal surface in order for porcelain to adhere was recognized in the 1960's. U.S. Pat. No. 4,205,982 discusses the role of base metals in porcelain bonding.
4. A solidus temperature higher than the firing temperature of the porcelain. The solidus temperature of an alloy is the point at which the alloy begins to melt. If this temperature is lower than the porcelain firing temperature, then the alloy will melt rendering it useless. U.S. Pat. No. 4,205,982 also discusses this problem.
Over the years, however, the market place has seen various developments which placed additional demands upon the alloys. The driving force for such developments has always been to improve the aesthetics of the restorations.
Thus, the demand for more economical alloys in the 1970's resulted in more silver being used in the alloys; but silver caused a discoloration in the porcelain. U.S. Pat. No. 4,123,262 and U.S. Pat. No. 4,387,072 eliminated the need for silver in the alloy composition.
Alternate materials and new porcelains were developed to be more life-like, and modified alloys, such as those taught by U.S. Pat. No. 5,462,437 were needed to accommodate the new porcelain.
The base metals used in the alloys sometimes produced a dark oxide, however, and it can be difficult to mask such oxides with the porcelain firings. The final restoration can in such circumstances therefore be left with an unsightly dark line at the porcelain-metal junction. U.S. Pat. No. 5,431,875 discloses a particular mix of base elements which is said to produce a light colored oxide on a palladium alloy.
Recent concerns in the market place therefore relate to the biocompatibility of the alloy as well as improved alloy-porcelain aesthetics.
German Patent DE 44 19 408 C 1 teaches of an alloy containing 95-98% gold, 1-4% titanium and 0.05-1.5% of the further elements Re, Rh, Ru, Ir and/or Ta. This alloy is said to be very biocompatible and to also have excellent aesthetics, due to its high gold content. Although the titanium content adds a degree of hardness to the alloy, it causes difficulties in reusing (i.e., recycling) the alloy. The titanium present in the alloy oxidizes so rapidly that most of it is depleted after a single casting. This severely limits the amount of alloy that can be recycled, making the restoration more expensive to produce. Normally, up to 50% of the metal in a restoration is recycled alloy.
It is therefore the object of this invention to provide a novel alloy that is at the same time economical, biocompatible and has good aesthetics.